Paramyxoviruses are enveloped negative-strand RNA viruses that are significant human and animal pathogens. Human Respiratory Syncytial Virus (hRSV, RSV) belongs to the family Paramyxoviridae, subfamily Pneumovirinae. Two subtypes, type A and type B, have been identified and are a major cause of severe and sometimes even fatal respiratory infections in children less than 6 months of age. Adults with underlying diseases, such as COPD, asthma, cancer, immunocompromised status, including HIV or post transplantation, are also at risk of developing severe RSV infection. 15% of annual hospitalizations in adults over 50 years due to acute respiratory infection are caused by RSV. In the United States, RSV causes more than 100,000 hospitalizations annually, and it is estimated to cause about 160,000 deaths globally each year. Currently there is no vaccine for RSV, and a trial with a formalin-inactivated virus was associated with increased disease severity in infants upon infection with RSV. Other family members including Human Metapneumo Virus (hMPV) and Human Parainfluenza Virus (hPIV) are also responsible for acute respiratory illness similar to hRSV.
The hRSV genome is a single-stranded negative-sense RNA molecule of approximately 15 kb that encodes 11 proteins. Two of these proteins are the main surface glycoproteins of the virion. These are (i) the attachment (G) protein, which mediates virus binding to cells, and (ii) the fusion (F) protein, which promotes both fusion of the viral and cell membranes at the initial stages of the infectious cycle and fusion of the membrane of infected cells with those of adjacent cells to form characteristic syncytia. The attachment protein G binds cellular surface receptors and interacts with F. This interaction triggers a conformational change in F to induce membrane fusion, thereby releasing the viral ribonucleoprotein complex into the host cell cytoplasm.
Monoclonal antibodies against the F protein or the G protein have been shown to have neutralizing effect in vitro and prophylactic effects in vivo. See, e.g., Beeler and Coelingh 1989, J. Virol. 63:2941-50; Garcia-Barreno et al., 1989, J. Virol. 63:925-32; Taylor et al., 1984, Immunology 52: 137-142; Walsh et al., 1984, Infection and Immunity 43:756-758; and U.S. Pat. Nos. 5,842,307 and 6,818,216. Neutralizing epitopes on the F glycoprotein were originally mapped by identifying amino acids that were altered in antibody escape variants and by assessing antibody binding to RSV F-derived peptides. These studies demonstrated neutralizing antibodies are often targeted to two distinct linear epitopes. See Graham et al., 2015, Curr Opin Immunol 35:30-38 for a review of the antigenic sites for the pre-fusion and post-fusion F forms. Antigenic site II (also called site A) includes residues 255 to 275 and is the target of palivizumab (SYNAGIS®, AstraZeneca). This epitope was predicted to be conformationally dependent, and the structure of a more potent derivative of palivizumab in complex with this epitope revealed that the linear epitope adopts a helix-loop-helix conformation. Antigenic site IV (also called site C) includes residues 422 to 438 and is the target of antibodies MAb19 and 101F. This epitope is C-terminal to the cysteine-rich region and is part of domain II, which in homologous paramyxovirus F glycoproteins remains structurally unchanged between pre- and post-fusion conformations. 5C4, AM22 and D25 delineate an epitope designated as site 0 which is only present on the pre-fusion F protein and were 50 times as potent as palivizumab. See McLellan et al., 2013, Science 340:1113-1117; International Patent Application No. WO 2008/147196 and U.S. Pat. No. 8,568,726. Other hRSV antibodies are described in International Patent Application Nos. WO94/06448 and WO92/04381 and U.S. Pat. No. 8,221,759.
An RSV vaccine for active immunization, if available, could not be utilized for the treatment of newborn babies with immature immune systems or patients who are immunosuppressed. In patients where prophylactic passive immunotherapy is required, as a result of a more chronic form of disease, current therapy is mediated via periodic intravenous inoculation of human IgG prepared from pooled plasma. This type of therapy, due to the low titers of neutralizing anti-RSV antibodies, involves a large quantity of globulin (e.g., 0.75 gm per kg) and consequently requires administration intravenously, in a clinic or hospital, over a lengthy period (2 to 4 hours), on a monthly basis during the high risk months (fall, winter and early spring).